Compatability with substrates and suitability for the intended use environment are important considerations in the choice of an adhesive for many applications, particularly for advanced or high performance uses.
For example, if a structure comprising an aluminum alloy bonded to a sheet of graphite fiber composite is fabricated for use at room or ambient temperatures, cure or post cure of a thermosetting resin adhesive at elevated temperatures, e.g. greater than 50.degree.C, will produce a final product which is warped as a result of differential thermal expansion between the aluminum alloy and the graphite fiber composite when the structure is cooled.
There are, of course, numerous examples of materials with inadequate thermal, chemical, and/or physical properties for specific uses. For example, sensitivity to moisture, particularly at elevated temperatures, is a recently noted deficiency of epoxy adhesives. Hydrolytic instability has also been encountered in silicone adhesives. Some thermosets are too rigid for use in areas of high torsion or flex, whereas some linear, thermally stable resins, such as polyquinoxalines, show unwanted thermal plasticity within their intended use range.
If a bonded joint does not fail cohesively, in the adhesive or adherend, the break generally occurs in an area called the "weak boundry layer", rather than at the adhesiveadherend interface. The weak boundry layer is a region near the interface where unrelieved stresses develop during the formation of the joint. These stress concentrations are generally the weakest link in the bond and serve to reduce its ultimate strength. Some of the principle causes for the development of stress concentrations are formation of bubbles, voids or inclusions because of high initial viscosity, loss of solvent, poor wetting of the surface areas by the adhesive, differential thermal expansion of adhesive and adherend, differential thermal expansion of dissimilar adherends, volume changes due to phase change polymerization during the cooling of a melt, and impurities or deleterious coatings which adversely alter surface characteristics.
The extent and strength deficiencies of the weak boundry layer may be alleviated, or in some cases eliminated, by changes of adhesive formulation, application and cure techniques, and adherend surface characteristics. For example, excessive viscosity may be reduced by the addition of plasticizer or solvent. Wetting may be improved by the addition of a surfactant to reduce the surface tension of the adhesive. Conversely, the surface energy of the adherend may be increased by chemical or radiation modification of the surface. Elastic stresses which may develop at points of contact between voids during cure under temperature and pressure may be reduced by annealing the joint under load.
Although the techniques for alleviating specific causes of stress concentrations are effective in many cases they often compromise other desirable properties. Plasticizers tend to reduce the cohesive strength of the polymeric materials, and the use of solvents for viscosity reduction may result in formation of bubbles during evaporation. Surfactants may reduce cohesive strength by also acting as plasticizers. Vigorous adherend surface treatments may result in local weak points or the creation of a new form of weak boundry layer extending into the body of the solid.
A more basic approach to the elimination of stress concentrations and weak boundry layers is the modification or tailoring of the molecular structure of the adhesive resin itself to correct deficiencies in wetting, viscosity, strength, and differential thermal expansion.